Aromatic sulphonation III: Sulphonation of benzenesulphonic acid and m‐toluenesulphonic acid in fuming sulphuric acid

The kinetics of the sulphonation of benzenesulphonic acid and m‐toluenesulphonic acid have been measured in oleum, containing 0.7 to 43.6% of sulphur trioxide The composition of oleum solutions is expressed in the percentage by weight of sulphur trioxide, present in excess of the composition H2SO4. The term concentration will be used for the different species present in oleum Solutions only. at 25°. The reaction is first order with respect to the sulphonic acid. The kinetics are shown to be consistent with the sulphonation mechanism proposed by Brand 1, in which the reaction proceeds by a stepwise attachment of SO3 and H+ to the substrate, followed by the loss of a proton from the benzene ring. Sulphonation of the protonated sulphonic acid cannot be ruled out. Its contribution to the total rate of sulphonation is regarded as small in view of its electrophilic nature.